Wheel fairings can be designed to have very low aerodynamic drag. However, on the ground at least, a wheel fairing must allow the tire to protrude from the fairing. In the air, this is a significant part of the drag of the entire airplane. Builders of racing airplanes go to great effort to minimize the amount of tire extending below the wheel fairing and to minimize the gap between the edge of the fairing and the surface of the tire. The result is a wheel fairing that is damaged (or destroyed) by roughness or discontinuities in runways or taxiways, or by gravel becoming wedged between the tire and the opening in the fairing. Such problems are acceptable in a race plane for the sake of gaining a bit of speed. Such problems are not acceptable in a general purpose airplane. A considerable improvement over the best wheel fairings flying today could be achieved by adding a door, or doors, that close in flight to completely enclose the wheel assembly. This has not been done, probably because it is so difficult to devise a practical implementation of such a fairing.
In most small airplanes, the landing gear is mounted on legs extending outward and downward from low on the fuselage. These gear legs are connected to the inboard end of the axle. The wheel fairings are mounted to the gear legs. Conventional wisdom is that such wheel fairings should have a cross section that is roughly oval shaped for most of the length of the fairing, which should end in a vertical knife-edge at the rear. On low wing airplanes with the gear legs attached to the wings, the gear leg could end in a fork (like that holding the front wheel of a bicycle), which gives much better strength to weight ratio than a cantilevered axle attachment. Existing wheel fairings for such airplanes still have the same general shape as that just described. This is not optimum. An optimum fairing has a ridge on the top extending from the gear leg to the aft end of the wheel fairing.
Aerodynamic drag can be divided into two general categories, pressure drag and friction drag. There is an optimum length for a fairing intended to minimize the total drag on a body moving thru the air. At low speeds (Reynolds number below about 100,000), the optimum ratio of the length of the fairing to its maximum width is about 5:1. If the fairing is made shorter, pressure drag will increase faster than the surface friction is reduced. Conversely, if the fairing is made longer, friction drag increases faster than pressure drag is reduced. The situation changes at high speed (Reynolds numbers above about 1 million). Turbulence at these speeds significantly increases friction drag relative to pressure drag, and optimum fairings are shorter,
Wheel fairings on modem, high-performance airplanes flying 100 m/s and faster, are operating at Reynolds numbers of 5 to 10 million. These will have turbulent air flow over much of the wheel fairing, certainly on the bottom, and most likely everywhere aft of the widest part of the fairing. Drag in turbulent flow is very complicated and there is no general solution for the optimum length to width ratio. Since friction drag increases relative to pressure drag, wheel fairings should have a length to width ratio well under 5:1 and in some cases it should approach 3:1. Apparently designers of wheel fairings have taken the published number of optimum L/W=5 without recognizing that the wheel fairings are far above the Reynolds number where that L/W ratio is applicable. Even wheel fairings on airplanes of 60 or 70 years ago were operating at Reynolds numbers of a few million, so this has been wrong from the start. The shortest wheel fairings presently available are manufactured by Klaus Savier, Santa Paula, Calif. His fairings have a length to width ratio of 4:1. Wheel fairings on high performance airplanes would have less total drag if they were shorter than this.
A cursory look at the wheel fairings on formula 1 planes at the Reno Air Races, for example, shows that most fairings are still shaped for laminar flow. Careful measurements reveal that the shortest fairings have a length to width ratio of 4.0 and L/W ratios of 5 and more are common. Discussions with pilots, crew chiefs, and designers reveals an attitude that the fairing should look good (or look like the plane over there that goes fast). It seems that no one has considered the increased drag caused by turbulence over the wheel fairings on planes traveling at the speeds presently being achieved.